Cold Joint In Slab

[Brooklyn2012]

I have an 8" thick slab with a 5'-0" span over a utility chase. The contractor was replacing some concrete and they saw-cut completely through the slab about 4" short of the side wall. Slab is drive way for truck traffic.

The proposed repair is to dowel in some #6 bars @6" OC and re-pour the edge.

I checked the dowel as an embed using Profis, and the capacity is about 3 kips and controlled by side break out.

I checked it using shear friction and the capacity is about 19 kips.

I read previous threads about full development length. If I use an epoxy and embed long enough to develop the rebar is that valid?

If I have to use development length, then I need to replace the slab.

My "feeling" is that the embed analogy is conservative and that the shear friction check seems like a lot of capacity. I don't have a lot of experience in this type of repair. In the past when this occurred, we used permanent post shores in the pit. I may do the same here.

I appreciate any advice you all may have.

 

[SlideRuleEra]

...8" thick slab with a 5'-0" span over a utility chase.
The contractor was replacing some concrete and they saw-cut completely through the slab...
Slab is drive way for truck traffic.

Replace the slab.
Contractor pays.

Edit: This is much a contract issue as it is technical:

1) Cutting thru a slab that thick is not just an "error", it's irresponsible, IMHO.

2) Accurately drilling aligned, numerous, properly spaced holes, about 1" diameter, deep into the edge of a reinforced slab 8" slab is going to be a challenge. If the holes are aligned "perfectly" they have only 3 1/2" cover, top and bottom. Drilling into existing rebar will insure the holes are not aligned or spaced as you intended.

3) Why would the Owner want to have a chopped-up slab that may cause problems in the future?

 

[canwesteng]

The bar should be fully developed, but not necessarily lap spliced. You'd have to consider where any flexural loads are going, that is less clear to me. If you're out of development length use more, smaller bars. I'm not sure how you plan to develop the bar with epoxy in a shorter distance than the development length. As an anchor, they will surely fail in breakout well before they reach yield.

 

[EngDM]

Not sure where you are located, but the Canadian concrete code has allowance to reduce development length based on the actual area of steel you require to resist the load. That's to say if your loads are low enough in the bar, you could get away with a lower development length and remain compliant.

I agree with SlideRuleEra however, replace the slab. Hopefully then the contractor won't make this mistake in the future.

 

[Brooklyn2012]

I think i am asking a more fundamental question. It seems to me that the anchor break out analogy is valid. It is extremely conservative because all of the load goes through the anchor. In shear friction, a large portion of the shear goes through the concrete. If you roughen the concrete and use a bonding agent, the shear transfer is more effective.

So why don't we worry about rebar break-out the end of a slab if it is monolithically poured? Even though we acknowledge that the concrete is going to crack.

Is the difference because the rebar is continuous or properly lapped?

Is it because the slab will crack at a different location than where maximum shear occurs?

What if the slab is designed as fixed end? Then you would expect a crack at a location of high moment.

 

[Brooklyn2012]

I do not know how to respond to individual responses directly in the posts, so I will do it here.

EngDM, I read in an earlier post that ACI does NOT allow for the area reduction with regard to shear friction. I still need to verify that and read the appropriate code section.

SlideRuleEra, I appreciate your comment. Sometime as engineers, our client is the guy who cut through the slab. Im not going to do anything unsafe. My litmus test is "would I stand under it while slightly overloaded?" Or maybe more appropriate, "would I let someone else stand under it".

"Hold Paramount the safety, health, and welfare of the public".

I am just trying to gain a comprehensive understanding of the situation. I may need to convince my client that what they want to do will not work.

 

[SlideRuleEra]

I am just trying to gain a comprehensive understanding of the situation.

Slab is drive way for truck traffic.

What loading assumption is being used for design?
Perhaps AASHTO HL-93 Live Load?

 

[Smoulder]

Shear friction design needs yield unless its been changed. For ductility. Otherwise the agg pushing the dhear plane apart will fail the bond, no clamp force, sudden collapse. Need on both sides so the new concrete side could be as big a priblem.

Also check reduced lap rqmt. Not aalowed in AU so we need >dev length in this situation.